JP2003049482A - Connection structure between column and beam, its connection method, and splice plate used for it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a connection structure between a column and a beam, its connection method, and a splice plate using it causing no buckling or local deformation even by a compressive stress applied to the splice plate connecting the column and beam together, reducing a cost, and exhibiting good construction workability. SOLUTION: This connection structure between the column and the beam is provided with a top plate 2 having a shape substantially equal to a plane shape of an H-beam column 1 and connected to its column head part, lower flange gusset plates 4a and 4b connected to the parts matching the lower flanges of the beams 8a and 8b connected to the top plate 2 in the strong axis direction and in the weak axis direction on at least one side of the H-beam steel column 1, and the splice plate 10 constructed of a connection part 11 connected to the top plate 2 and a plurality of connection pieces 12 and 13 extended from the connection part to be connected to the upper flanges of the beams 8a and 8b.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a column-beam joining structure at the top of a steel frame structure, a joining method therefor, and a splice plate used therefor. More specifically, the present invention relates to a comparison using H-shaped steel columns. TECHNICAL FIELD The present invention relates to a joint structure of a column and a beam in the uppermost column head in a steel frame structure having a two-dimensional rigid frame structure with a relatively low layer, a joining method therefor, and a splice plate used therefor.

[0002]

2. Description of the Related Art For example, the uppermost pillar head of each pillar of a low-rise steel frame of the second to third floors is provided with a roof on its upper part, and the roof has various shapes. It is built on top of the top steel structure. Therefore, it is necessary that the tops of the pillars have structural details that do not protrude as much as possible so that the overall height can be suppressed, the structure of the roof can be simplified, and that heat insulation and rain warding can be achieved.

In the case of a relatively small-scale steel frame structure and the column is an H-shaped steel, an end plate is often used for joining the beams. In this type of structure, the amount of protrusion of the column top from the beam upper surface is large. It is large (usually about 80 mm to 100 mm), which makes the installation of the roof cumbersome. As a technique for solving such a problem, there is a device described in Japanese Utility Model Laid-Open No. 5-3403 (Prior Art 1).

The column-beam joint structure of the column head according to the present invention is
In a pair of flanges of the H-shaped column, a T-shaped beam joint material fixed to the outer surface slightly below the capital by bolts, and a top plate bolted to the pillar web at the column head by angle members, and the beam joint It is composed of a plate and a beam joined to the top plate by bolts.

Further, as an example of using a square steel pipe for a column, there is an invention described in Japanese Patent Laid-Open No. 3-17327 (Prior Art 2). The beam-to-column joining structure according to the present invention has a structure in which a support metal having an inverted L-shaped cross section is disposed on the side surface of the upper end of the column to fix a vertical piece of the support metal to the column, Fix the vertical piece of the V-shaped bracket, attach the top plate to the upper end surface of the pillar, and place the bottom plate of the top plate on the bottom piece of the base on the horizontal piece of the supporting metal piece to the side of the pillar. The end of the beam is fixed to the receiving piece of the supporting metal and the horizontal piece of the receiving metal fitting so that the beam is joined to the column.

[0006]

In the prior art 1, the beam joined in the weak axis direction (web direction) of the H-shaped column is of a steel frame using the H-shaped column having directional cross-sectional performance. As a general rule, the flange is not joined to the column, but only the web is joined to the column, so the column has a non-ramen structure in the weak axis direction and cannot be a two-way frame structure. Further, the lower flange of the beam in the strong axis direction (flange direction) of the H-shaped steel column is an end plate joint using a T-shaped beam joint material, but the beam joint material protrudes from the side surface of the column. Therefore, finishing of the ceiling and columns (enclose the columns with boards, etc.)
Etc. will be adversely affected.

Further, although there are advantages that the welding work is relatively small and the accuracy control is easy, there is a problem that the number of joining bolts is large and reinforcement is required on the column side, and at least the beam joining material is required. Since the end of the beam is separated from the outer surface of the column by the thickness of the vertical piece, the top plate that joins the upper flange of the beam and the column is susceptible to buckling and local deformation when subjected to compressive stress. There's a problem.

[0008] In Prior Art 2, since a square steel tube having a closed cross section is used for the column, a two-way rigid frame structure can be obtained, but a backing and a nut for fixing a bolt are provided inside the column. It requires some ingenuity and is troublesome. Further, since the lower flange of the beam is fixed to the T-shaped receiving metal fitting provided on the outer surface of the column, a gap is formed between the column side end of the upper flange of the beam and the outer surface of the column. The distance between the beam side bolt and the pillar side bolt of the receiving piece of the top plate becomes large, and there is a possibility that buckling or local deformation may occur when the beam receives compressive stress. Therefore, in order to prevent such buckling and local deformation, it is necessary to provide reinforcing pieces, fixing bolts, etc., which increases the number of members, complicates the structure, and increases the cost.

Further, since the T-shaped receiving metal fitting is used, the receiving piece is correspondingly long, the whole top plate is large and heavy, and not only the cost is increased but also the workability is improved. There is also a problem in terms of.

The present invention has been made in order to solve the above problems, and has a simple structure, and there is a possibility that buckling or local deformation may occur even if a splice plate for joining a column and a beam receives a compressive stress. It is an object of the present invention to provide a joint structure of a column and a beam that forms a two-way rigid frame structure that can reduce costs and has excellent workability, a joining method therefor, and a splice plate used therefor.

[0011]

(1) A column-beam joining structure according to the present invention is a top plate joined to a column head of the H-section steel column in a shape substantially equal to the planar shape of the H-section steel column. When,
A lower flange gusset plate joined to a position corresponding to a lower flange of a beam joined to at least one of the H-shaped steel columns in the strong axis direction and the weak axis direction, and a joint portion joined to the top plate And a splice plate including a plurality of joining pieces extending from the joining portion and joined to the upper flange of the beam.

(2) Further, in the column-beam joint structure according to the present invention, the H-shaped steel column between the top plate and the lower flange gusset plate provided in the H-shaped steel column of the above (1) is used.
A web gusset plate to which a web of beams is joined is provided on a web of shaped steel columns or a web and a flange.

(3) The method of joining columns and beams according to the present invention is
A top plate is welded to a column head in a factory or the like, and a plurality of H-shaped steel columns with a lower flange gusset plate welded to a position corresponding to a lower flange of a beam to be joined are erected at predetermined positions. A step of placing both ends of the beam on the lower flange gusset plate of the adjacent H-shaped steel column and temporarily fixing the beam, and a splice plate on the top plate provided on the H-shaped steel column. And tentatively fastening it to the top plate and the upper flange of the beam, and joining the tentatively fastened lower flange gusset plate and the lower flange of the beam, and the splice plate and the top plate and the upper flange of the beam with bolts, respectively. And a process.

(4) In addition, in the method of joining the column and the beam according to the present invention, in the above (3), the step of temporarily fastening the web of the beam to the web gusset plate provided on the H-shaped steel column and then joining It is equipped with.

(5) The splice plate according to the present invention is the splice plate used in any of the above (1) to (4), and is joined to the top plate provided on the head of the H-shaped steel column. A joint portion and a plurality of joint pieces that extend from the joint portion and are joined to the upper flange of the beam and have a width substantially equal to the width of the upper flange, and R are provided at the corners of the joint portion of the joint piece. It is processed or diagonally cut.

(6) Further, the splice plate according to the present invention is provided with a substantially circular through hole at the central portion of the joint portion of the splice plate of the above (5).

[0017]

1 is a perspective view of a column-beam joint structure according to an embodiment of the present invention, and FIG. 2 is an exploded perspective view of FIG. In both figures, reference numeral 1 denotes a column (hereinafter referred to as an H-shaped steel column) that constitutes a steel frame made of a steel material having an H-shaped cross section.
Reference numeral 2 is a top plate made of a steel plate joined to the head of the H-shaped steel column 1 by welding, and its outer shape is a plane outer shape that connects the flanges of the H-shaped steel column 1 (hereinafter, referred to as plane shape).
A plurality of bolt insertion holes 3 are provided with a shape substantially equal to or slightly larger than.

A lower flange joint 4a is welded to the flange (strong axis direction) at right angles to the lower flange of the beam 8a, which will be described later, below the column head of the H-shaped steel column 1 by welding. Gusset plate for a lower flange (hereinafter, referred to as a lower flange gusset plate), the width of the tip portion is substantially equal to the width of the lower flange of the beam 8a, and the base portion is formed in a trapezoidal shape having a wider width than this. A bolt insertion hole 5 is provided.

Numeral 4b denotes both sides of the web in the direction of the weak axis of the H-shaped steel column 1, and at a position corresponding to the lower flange of the beam 8b described later, a gusset plate for joining the lower flange joined to the web and the flange by welding ( Hereinafter, referred to as a lower flange gusset plate), the lower flange gusset plate 4a in the strong axis direction from the end of the flange of the H-shaped steel column 1
Projected in the same shape as that of the multiple bolt insertion holes 5
Is provided.

6a is a lower flange gusset plate 4a
Is a web gusset plate for joining the web of the beam 8a, which is welded to the flange of the H-shaped steel column 1 in the up and down direction at right angles to the above. The web gusset plate 6a is not essential to the present invention and may be omitted. Similarly, 6b is located above the lower flange gusset plate 4b so as to be orthogonal to the vertical direction of the web of the H-shaped steel column 1 and its tip end is located substantially flush with the tip end of the flange of the H-shaped steel column 1. It is a gusset plate for the web welded to. The web gusset plate 6b may be omitted depending on the situation. 7 is a gusset plate for both webs 6a, 6
It is a plurality of bolt insertion holes provided in b.

Reference numeral 8a is a beam made of a steel material having an H-shaped cross section that is joined in the strong axis direction of the H-shaped steel column 1, and a bolt insertion hole 9a for joining a splice plate 10, which will be described later, is attached to its upper flange. The lower flange is provided with a bolt insertion hole 9b for joining to the lower flange gusset plate 4a, and the web is provided with a bolt insertion hole 9c for joining to the web gusset plate 6a. Reference numeral 8b is a beam that has the same structure as the beam 8a and is joined in the weak axis direction. Bolt insertion holes 9a, 9b, and 9c are provided in the upper flange, the lower flange, and the web, respectively.

A splice plate 10 is joined to a top plate 2 provided on the head of the H-shaped steel column 1 and is joined to the upper flange of the beam 8a in the strong axis direction and the upper flange of the beam 8b in the weak axis direction. Then, as shown in FIG. 3, in the joint portion 11 to the top plate 2, the joint piece 12 joined to the upper flange of the beam 8a in the strong axis direction and the joint pieces 12 provided on both sides orthogonal to this are provided. A joint piece 13 joined to the upper flange of the beam 8b of the beam 8b is extended, and these joint pieces 12 and 13 are formed to have a width substantially equal to the width of the upper flange of the beams 8a and 8b, and these adjacent joints are formed. A portion of the pieces 12 and 13 that overlaps with the top plate 2 (a corner portion of the joint portion) is rounded 14 for preventing stress concentration, or is diagonally cut 14a as shown in FIG. 3B. . For this reason,
The joint portion 11 of the splice plate 10 is formed smaller than the upper surface (shown by the broken line) of the top plate 2.

The joint plate 11 has a top plate 2
A bolt insertion hole 15 corresponding to the bolt insertion hole 3 and a bolt insertion hole 16 corresponding to the bolt insertion hole 9a of the upper flange of the beams 8a and 8b. There is. Reference numeral 17 is a substantially circular through hole provided in the central portion of the joint portion 11, but this may be omitted. In the above description, the case where three joining pieces 12 and 13 are provided at 90 ° intervals on three sides of the joining portion 11 of the splice plate 10 is shown, but both sides of the H-shaped steel column 1 in the strong axis direction and the weak axis direction are shown. When connecting the beams to the splice plate 10 respectively,
It is sufficient to provide cross-shaped joint pieces at 90 ° intervals on all four sides of the joint portion 11 of FIG. 11. Further, when the beam is joined to only one side in the strong axis direction and the weak axis direction, the joint portion 11 has 90 °. Two joining pieces may be provided at intervals.

Referring again to FIGS. 1 and 2, 18 is a web gusset plate 6b joined to the web in the weak axis direction,
It is a splice plate for joining the web of the beam 8b, and bolt insertion holes 19 are provided corresponding to the bolt insertion holes 7 and 9c provided in the web gusset plate 6b and the web of the beam 8b. The web gusset plate 6b and the splice plate 18 may be previously integrally joined by welding, or may be formed of a single steel plate.

Next, an example of the construction procedure of the present invention comprising the above members will be described. First, in a factory or the like, as shown in FIG. 4, the top plate 2 is joined to the column head of the H-shaped steel column 1 by welding, and the lower flange gusset plate is attached to one flange in the strong axis direction of the H-shaped steel column 1. 4a
Further, the lower flange gusset plate 4b is welded to both surfaces and the flange of the web in the weak axis direction. Then, the gusset plates 6a and 6b for the web are joined by welding to one flange of the H-shaped steel column 1 and both sides of the web to manufacture the column.

The plurality of pillars manufactured as described above are transported to a construction site and are erected at predetermined positions. Then, the beam 8a in the strong axis direction and the beam 8b in the weak axis direction are lifted by a crane or the like, and both ends thereof are placed on the gusset plates 4a, 4b for the lower flange of the H-shaped steel column 1 adjacent to each other. Insert the bolts into the bolt insertion holes 5 and 9c and temporarily fix them.

Next, the splice plate 10 is placed on the top plate 2, and the joining pieces 12 and 13 thereof are placed on the beams 8a and 8b, respectively, for alignment, and the joining portion 11 and each joining piece 12 and 13 is temporarily fixed to the top plate 2 and the upper flanges of the beams 8a and 8b with a small number of bolts. Further, the web gusset plate 6a is temporarily fixed to the web of the beam 8a with a small number of bolts, and the web gusset plate 6b is used by using the splice plate 18.
The web of beams and beams 8b is temporarily fixed with a few bolts.

Finally, after inserting the bolts into all the bolt insertion holes and confirming the condition of the entire frame, if all the bolts are fully tightened and fixed, the H of the uppermost part of the steel frame by the bidirectional rigid frame structure is fixed. Joining of the shaped steel column 1 and the beams 8a and 8b is completed.

In the above description, the web gusset plates 6a and 6b are provided in both the strong axis direction and the weak axis direction, but in the strong axis direction, the beam 8a is replaced by the H-shaped steel column 1.
Since it can be joined close to or in contact with the flange of
If the shearing force is small, the web joining is unnecessary, so the strong axis web gusset plate 6a may be omitted, and depending on the situation, the weak axis web gusset plate 6b may be omitted. You can

FIG. 5 is a perspective view showing another example of the present invention. In this example, a hanging metal fitting of, for example, an inverted U shape is provided at the central portion of the top plate 2 provided on the column head of the H-shaped steel column 1. 20 is attached by welding or the like, and when the splice plate 10 is joined to the top plate 2, the upper part of the hanging metal fitting 20 is exposed through the through hole 17. In this case, the top of the hanging metal fitting 20 is located substantially flush with the top of the fixing bolt of the splice plate 10.

With the above construction, the beams 8a,
When the H-shaped steel column 1 (state shown in FIG. 4) before joining the 8b and the splice plate 10 is lifted by a crane or the like, this hanging fitting 20 can be used. Also,
After mounting the splice plate 10, the top of the hanging metal fitting 20 is located substantially on the same plane as the fixing bolts of the splice plate 10, so there is no hindrance to the construction after that. Construction can be continued without cutting off. Further, it can be used when attaching a support material for roof construction in advance. Further, the splice plate 10 can be lifted up on the ground in a state of being temporarily fastened to the top plate 2, and thus the construction speed can be further improved.

In the present invention constructed as described above,
The top plate 2 provided on the column head of the H-shaped steel column 1 has substantially the same shape as the plane shape of the H-shaped steel column 1, and it hardly projects from the outer peripheral surface of the H-shaped steel column 1, and Since the flange gusset plates 4a and 4b are not provided with end plates, the beams 8 in the strong axis direction and the weak axis direction are formed.
The ends of a and 8b can be installed close to or in contact with the outer surface of the H-shaped steel column 1.

As a result, the projection length of the lower flange gusset plates 4a, 4b and the web gusset plates 6a, 6b attached to the H-shaped steel column 1 in the direction of the beams 8a, 8b can be shortened. Since the size (packing style) of the H-shaped steel column 1 (FIG. 4) to which the elements are attached can be made compact and lightweight, the manufacturing cost can be reduced and the transportation efficiency can be improved. Further, as described above, since the beams 8a and 8b can be installed close to or in contact with the outer surface of the H-shaped steel column 1, there is a gap between the end of the beam and the column as in the prior art, The problem of buckling or local deformation of the splice plate can be solved.

Further, conventionally, a gusset plate for joining a beam web to a column is generally attached to the pillar, and the web gusset plates 6a and 6b are provided in the present invention as described above. The gusset plates 6a and 6b can be omitted when the shearing force of the beams 8a and 8b is small. In particular, in the strong axis direction, since the upper and lower flanges of the beam 8a and the tip of the web are installed close to or in contact with the outer surface of the H-shaped steel column 1, even if the shearing force of the beam 8a is large to some extent The gusset plate 6a can be omitted.

Further, the splice plate 10 for joining the top plate 2 provided on the column head of the H-shaped steel column 1 and the upper flanges of the beams 8a and 8b to the portion (joint portion 11) to be joined to the top plate 2. A plurality of joining pieces 12 and 13 for joining the upper flanges of the beams 8a and 8b are provided.
Widths 2 and 13 are formed to be substantially equal to the widths of the upper flanges of the beams 8a and 8b, and overlap with the top plate 2 (joint 1
Since the R processing 14 or the oblique cutting processing 14a is applied to the corner portion of the portion orthogonal to 1, the stress concentration in this portion can be prevented. Further, as a result, the splice plate 10 is lighter and smaller in size and has better workability than the conventional top plate (corresponding to the splice plate 10 of the present invention) 10a having a large weight and poor workability as shown in FIG. Can be realized.

Further, if the through hole 17 is provided at the center of the joint portion 11 of the splice plate 10, the weight can be further reduced, and the hanging metal fitting 20 provided on the top plate 2 of the H-shaped steel column 1 can be used. The top part of the hanging metal fitting 20 that is housed in the through hole 20 and exposed from the top plate 2
By making the height almost equal to the top of the bolt for fixing the roof, not only does the construction of the roof be hindered, but also a support material for roof construction can be attached to the suspension fitting 20 in advance. it can.

In the construction, the upper flange gusset plates 4a, 4b are provided on the lower flange gusset plates 4a, 4b corresponding to the lower flanges of the beams 8a, 8b in both the strong axis direction and the weak axis direction of the H-shaped steel column 1. In addition, the beams 8a and 8b can be temporarily placed, and the hanging operation can be released earlier without performing positioning in the height direction. Therefore, not only the number of assembling steps can be shortened, but also the expensive wrecker cost can be saved.

As described above, in the present invention, only the tops of the splice plate 10 and the bolts for fixing the splice plate 10 to the top plate 2 are projected to the column head of the H-shaped steel column 1, so that the upper and lower flanges of the beam are formed. The roof construction is easier than the case of joining to the pillar by the end plate type. Further, as compared with the case where a conventional top plate corresponding to the splice plate 10 is used, provisional receiving of the beams 8a and 8b and the like can be performed easily, and a lightweight member having no extra reinforcing structure can be provided.

[0039]

The joint structure of the pillar and the beam according to claim 1 is H
The top plate joined to the column head of the H-section steel column in a shape almost equal to the plane shape of the section steel column, and the bottom of the beam joined to this in the strong axis direction and the weak axis direction of at least one of the H section steel columns A lower flange gusset plate joined at a position corresponding to the flange, a splice plate including a joint portion joined to the top plate and a plurality of joint pieces extending from the joint portion and joined to the upper flange of the beam. Equipped with
Since the end of the beam is installed close to or in contact with the outer surface of the H-shaped steel column, the structure is simple and construction is easy, and the splice plate does not cause buckling or local deformation at low cost. A joint structure can be obtained.

According to a second aspect of the present invention, there is provided a joint structure of a column and a beam, wherein the web of the H-shaped steel column or the web and the flange are provided between the top plate and the gusset plate for the lower flange provided on the H-shaped steel column. Since the web gusset plate to which the web of the beam is joined is provided in the above, it is possible to resist the large shearing force of the beam.

According to a third aspect of the present invention, there is provided a method of joining a column and a beam, wherein a top plate is welded to a column head in a factory or the like, and a lower flange gusset plate is welded to a position corresponding to a lower flange of the beam to be joined. And a plurality of H-shaped steel columns are erected at predetermined positions, a step of temporarily mounting both ends of the beam on the gusset plate for the lower flange of adjacent H-shaped steel columns, and an H-shaped Place the splice plate on the top plate provided on the steel pillar, and temporarily fix it to the top flange of the top plate and the beam, and the gusset plate for the lower flange and the lower flange of the beam that are temporarily fastened, the splice plate and the top plate, and Since the upper flange of the beam is joined by bolts, respectively, the same effect as in the case of claim 1 can be obtained, and the construction period can be shortened. .

Since the pillar-beam joining method according to the fourth aspect comprises the step of joining the beam web to the web gusset plate provided on the H-shaped steel column in the third aspect, the beam web is temporarily fastened. It is possible to obtain substantially the same effect as the case of claim 2.

The splice plate according to claim 5 is H
A joint portion joined to the top plate provided on the pillar head of the shaped steel column and a plurality of joint pieces having a width substantially equal to the width of the upper flange extended from the joint portion and joined to the upper flange of the beam, Since the rounded corner portion of the joint portion of the joint piece is rounded or obliquely cut, it is possible to obtain a splice plate which is lightweight and small in size and in which stress concentration is not likely to occur at the corner portion of the joint portion.

The splice plate according to claim 6 has a substantially circular through hole in the central portion of the joint portion according to claim 6, which contributes to further weight reduction and is provided at the head of the H-shaped steel column. The hanging metal fitting can be accommodated in this through hole, and the subsequent work can be performed without any trouble.

[Brief description of drawings]

FIG. 1 is a perspective view of a column-beam joint structure according to an embodiment of the present invention.

FIG. 2 is an exploded perspective view of FIG.

FIG. 3 is a plan view of the splice plate of FIG.

FIG. 4 is a perspective view showing a state where the H-shaped steel column of FIG. 1 is assembled in a factory or the like.

FIG. 5 is a perspective view of another example of a column-beam joint structure according to the present invention.

FIG. 6 is a plan view of an example of a conventional splice plate.

[Explanation of symbols]

1 H-shaped steel column 2 top plate 4a, 4b Lower flange gusset plate 6a, 6b Web gusset plate 8a, 8b beams 10 splice plate 11 joints 12,13 Joining piece 14 R processing 14a Diagonal cutting 17 through holes 18 splice plate 20 hanging metal fittings

Claims (6)

[Claims] 1. A top-column joint structure of a column and a beam at the top of a steel frame structure, the top plate being joined to the column head of the H-section steel column in a shape substantially equal to the planar shape of the H-section steel column, A lower flange gusset plate joined at a position corresponding to a lower flange of a beam joined to at least one of the H-shaped steel columns in the strong axis direction and the weak axis direction, and a joint portion joined to the top plate And a splice plate formed of a plurality of joint pieces extending from the joint portion and joined to the upper flange of the beam, and a joint structure of a pillar and a beam. 2. A web gusset plate in which a beam web is joined to the web of the H-shaped steel column or the web and the flange between the top plate provided on the H-shaped steel column and the gusset plate for the lower flange. The column-beam joint structure according to claim 1, further comprising: 3. A plurality of H-shaped steel columns in which a top plate is welded to a column head in a factory or the like and a lower flange gusset plate is welded to a position corresponding to a lower flange of a beam to be joined are predetermined. Standing up at a position, placing both ends of the beam on the lower flange gusset plates of the adjacent H-shaped steel columns and temporarily fixing them, and on the top plate provided on the H-shaped steel columns Place a splice plate on the top plate and the upper flange of the beam, and temporarily fix the lower flange gusset plate and the lower flange of the beam, the splice plate, the top plate and the upper flange of the beam. A method for joining a column and a beam, the method including joining with bolts. 4. The method for joining a column and a beam according to claim 3, further comprising the step of temporarily fixing the web of the beam to the web gusset plate provided on the H-shaped steel column and then joining the web. 5. The splice plate used in any one of claims 1 to 4, wherein a joint portion joined to a top plate provided on a column head of an H-shaped steel column and a beam extending from the joint portion. A plurality of joint pieces having a width substantially equal to the width of the upper flange joined to the upper flange, and R-cutting or oblique cutting processing is applied to the corners of the joints of the joining pieces. Splice plate. 6. The splice plate according to claim 5, wherein a substantially circular through hole is provided in the central portion of the joint portion.